Your MacBook may be slower because of these reasons:
1. The hard drive on the MacBook is full --- At this time, you only need to clean up the junk files on your hard drive. If you feel that it is a cumbersome task to manually find junk files, you can download the Mac cleanup tool to clean it up. I recommend you use iMyMac's Mac Cleaner. This software is relatively small and cleaned up.
2. Your macOS is too old --- Update to the latest version of macOS.
3. The startup is slowing you down --- You can manage your Startup Items.
4. Run too many programs in the background --- End the process using the Activity Monitor.
5. Your Mac hardware is too old --- You can consider upgrading your MacBook hardware.
6. Too many things on the desktop are still messy --- Clean up your MacBook's desktop and organize the files on your desktop into separate folders.
7. There are a lot of useless extensions in the browser --- Quickly remove the browser extension to make your Mac run faster.
8. There are a lot of outdated cache files in the MacBook---Just clean them up.
9. By default, your Finder is set to search for files within All My Files view. When files on your Mac are too many, the Finder slows down and opening each new window becomes a burden.
--- Now you change the default settings from “All My Files” to other more specific location.
10. Animations and rich graphics are extremely resource-hungry. --- Reduce MacBook animations, which can be modified under Preferences > Dock.
11. SMC-settings are corrupted --- Reset SMC to factory settings.
12. Spotlight overload --- Limit Spotlight's search area.
13. There are too many temporary files in memory --- Please restart your MacBook regularly!
14. FileVault weighs on memory --- Turn off FileVault encryption.
15. Broken or conflicting permissions --- Please repair disk permissions.
16. Piles of large and old files --- Clean up Downloads and Trash folders.
17. There are too many icons in the Preferences pane --- Just remove all the extra from Preference Panes and you can speed up your Mac.
Having said so many reasons, in fact, the most important thing is to make more space on your MacBook to make your MacBook run faster!